Migrate `pallet-indices` to umbrella crate #53

Job	Run time
preflight / preflight	1m 23s
preflight / ci-versions	50s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	7m 13s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	4m 59s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	22m 17s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	3m 58s
publish-benchmarks	0s
	40m 40s

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